feat(wallpapers-bing): restore preferred-region picker

[perlowja]

Adds a Bing Preferred Region SelectionRow to get_settings_widget() -- 14 options (All Markets, No Preference, plus 13 individual markets labelled "Region - Market"), reading/writing ~/.config/ncz-wallpaper/bing-markets, the same file cix-installer's 45-wallpaper-rotator.sh ncz-wallpaper-bing helper already reads.

Ports a complete, working implementation (singularity-shellfa5a38a, zoder-reviewed, full ninja+meson test pass at the time) that lived inline on the Desktop settings page before the plugin-based wallpaper manager (132efb7) deleted it as collateral damage of that rewrite -- it never got a new home in the plugin architecture.

Every market is always fetched and combined by the helper regardless of this setting; the picker only chooses whose caption/credit wins when the same photo is shared across regions.

Verified: full ninja -C builddir build succeeds (0 errors) against a fresh subproject checkout of libsingularity.

Assisted-by: Claude Code:claude-sonnet-5
AI scope: ported the deleted desktop_page.vala implementation into the plugin's settings widget, adjusted class/field placement for the plugin context.

AI assistance: disclosed

[perlowja]

@mirkobrombin bumping this one too - restores the preferred-region picker for the Bing wallpaper source.

[mirkobrombin]

Singularity must stay distro-agnostic: the plugin shouldn't write another distro's config (~/.config/ncz-wallpaper/bing-markets) or hardcode the market list. Get markets from the helper's markets command and persist the choice through the helper. Also: bing_markets_updating is never set, use string.ascii_down() instead of ascii_lower, and drop the cix-installer/zoder references from comments.